1. Field of the Invention
The present invention relates to an error signal generating circuit which can be applied to an optical disc reproducing apparatus.
2. Description of the Related Art
Generally in an optical disc reproducing apparatus, there has been known, as a method for detecting errors in the tracking direction of the optical disc, a so-called 3-beam method including a main beam used for controlling a digital data reading operation or focus controlling operation, and two sub-beams used for following the track. FIG. 1 shows an example of a circuit for generating a tracking error signal by the use of sub-beams and adjusting the position of the sub-beams on the optical disc.
In FIG. 1, photo diodes 1 and 2 detect reflected light of the sub-beams and generate a detecting signal in reply to the amount of the reflected light. After being amplified in amplifiers 3 and 4, the detected signal is input to LPFs 5 and 6 to get rid of any high-frequency component due to pits. The pits-free detected signals are then mutually subtracted in a difference signal generating circuit 7 to generate a difference signal which is used as a tracking error signal. The difference signal is phase-compensated in a servo phase compensating circuit 8 and is then applied to a driver 9 which corrects the position of the sub-beams on the optical disc by driving the tracking coil 10 to cause the difference signal to become zero.
In such a 3-beam method, when the sub-beams are accurately tracking on the optical disc, the amount of reflected light of the sub-beams is the same. This means that the magnitude of the detected signals from the photo diodes 1 and 2 is the same, so as not to render any difference signal. As a result, no operation for correcting the sub-beam position on the optical disc is carried out. In the case of any tracking error taking place, the amount of reflection of the sub-beams will become different, so that the difference signal generating circuit 7 generates a difference signal to correct the sub-beam position for accurately performing the tracking operation. In this manner, it is possible to always track stably and correctly on the disc.
In such a system, however, there has been a fear of generating a tracking error even if the sub-beams are in the correct positions on the optical disc because the opto-current conversion efficiencies of the pair of photo diodes 1 and 2 are different from each other.
To cope with the above-mentioned disadvantage, there has been carried out an adjustment in order not to generate any tracking error signal at a position where the sub-beams correctly track, by setting the feed-back resistor of the amplifier 4 as a variable resistor so as to vary the gain of the amplifier 4.
The characteristics of the variable resistor however tend to change depending on vibration or with the passage of time, so that sometimes, a tracking error signal may appear even if the sub-beams are on the correct positions on the optical disc.
Further, such a type of adjustment is carried out by the use of an adjusting disc during the manufacturing processes, in such a manner that the adjusting disc is rotated to adjust an average value of the signals, generated when the beams cross a plurality of tracks on the disc, to a reference value. In this method, however, for high-accuracy adjustment it requires an excessively long time, leading to a decrease of the manufacturing efficiency.